This invention relates to metal-to-ceramic seals, high voltage vacuum devices employing such seals and processes for fabricating them.
High voltage vacuum devices, e.g., vacuum tubes, vacuum switches, and the like, have been known for many years. Such devices contain electrodes surrounded by an envelope which isolates the electrodes from atmospheric conditions. The electrodes are supported by insulators and, frequently, the envelope of such a device serves an insulating function as well. The insulators and the envelope have frequently been fabricated from glass or glass-like materials. However, in some instances, it is desirable to fabricate the insulators and/or envelope from ceramic materials.
Such components, when manufactured from ceramic materials, have exhibited poor high voltage holdoff. As used herein, high voltage holdoff refers to the ability of a high voltage vacuum device containing two or more electrodes to have a high voltage electrical potential established between the electrodes without, e.g., arcing therebetween or along the interior walls of the device. Once such arcing occurs, a conductive path is frequently established along the interior wall of the device. In general, once such arcing occurs, the device will not subsequently function properly. This is a particular problem in small devices, where the distance between electrodes is small.
In contradistinction, high voltage vacuum devices wherein such components are fabricated of glass, or glass-like materials, generally exhibit good high voltage holdoff. This is due to the nature of the electric field gradient at the metal/glass joint or seal. As used herein, electric field gradient refers to the transition in electrical resistivity exhibited at the metal/glass or metal/ceramic joint or seal.
In metal-to-ceramic seals or joints, the transition in resistivity is abrupt. Thus, at the seal or joint, highly conductive metal is in contact with highly non-conductive ceramic. The electric field gradient is therefore not gradual. Prior art high voltage vacuum devices wherein an electrode member is embedded in the ceramic walls of the device have been attempted but have exhibited poor mechanical stability resulting from thermal stress due to the differences in coefficients of expansion of ceramic and metal.
Various prior art devices employ metal-to-ceramic seals which include, in addition to the metal and ceramic, an additional material. U.S. Pat. No. 3,355,564 discloses a vacuum circuit interrupter which includes a metal-to-ceramic joint which also includes a metallization layer of an alloy of molybdenum and manganese. It is applied to enhance the mechanical strength of the joint and effects negligible electrical gradient. U.S. Pat. No. 4,366,410 provides similar disclosure.
U.S. Pat. No. 3,854,827 discloses a metal-to-ceramic leadthrough which includes, in addition to the molybdenum conductor in the ceramic body, a glass-like material comprising MnO, SiO.sub.2 and Al.sub.2 O.sub.3. U.S. Pat. No. 4,334,628 presents a similar disclosure.
U.S. Pat. Nos. 2,174,375; 2,431,277; 2,450,780; 2,446,277, 2,836,935; 2,949,376; 3,062,981; 3,304,362; 3,371,406 and 3,343,515 further represent the state-of-the-art.
In general, prior art high voltage vacuum devices having ceramic-to-metal seals exhibit inadequate performance due to low high-voltage holdoff capabilities and poor mechanical stability.